A loudspeaker is a device which converts an electrical signal into an acoustic signal (i.e., sound) and directs the acoustic signal to one or more listeners. In general, a loudspeaker includes an electromagnetic transducer (also referred to as a “driver”) that receives and transforms the electrical signal into a mechanical vibration. The mechanical vibrations produce localized variations in pressure about the ambient atmospheric pressure, and the pressure variations propagate within the atmospheric medium to form the acoustic signal.
A loudspeaker including multiple transducers (or drivers) and a single horn is known. For example, U.S. Pat. No. 5,526,456, which is incorporated by reference herein, describes a loudspeaker including one or more low frequency drivers and one or more high frequency drivers that are coaxially arranged with respect to the centerline of the loudspeaker. The loudspeaker further includes a single horn, which acts as a waveguide for the sound produced by both the low and high frequency drivers. The present description uses the term “coaxial transducer” to refer to a set of two or more drivers (transducers) that are coaxially arranged, i.e., with one driver in front of, or on the same axis of, another driver.
The successful implementation of such coaxial transducers in loudspeakers, however, poses certain engineering challenges. Coaxial transducers have generally been designed for use in two-way, full-range, low Q systems. (Q, or the directivity factor, is the ratio of the intensity of a source at a given location, to the intensity produced at the same location by a point source (omnidirectional source) radiating the same acoustic power.) Referring to FIG. 1, a coaxial transducer 10 typically includes a cone-type mid-frequency (MF) driver 14 having a cone-shaped diaphragm 11 (for example, with the diameter of 8″, 10″, 12″, or 15″) and a high-frequency (HF) compression driver 16. As used herein, MF refers to a frequency range of about 200 Hz to 2 kHz, and HF refers to a frequency range over about 2 kHz. The HF compression driver 16 is mounted on the back of the MF driver's motor structure so that the HF driver 16 produces (or fires) HF acoustic signals through the center of the MF driver 14. To this end, the MF driver's pole piece is hollowed out and shaped to provide an initial horn 18 for the HF driver 16. The initial horn 18 (acting as a waveguide for the HF acoustic signals) terminates at the rear end 11a of the cone-shaped diaphragm 11, from which the cone-shaped diaphragm itself becomes a continuation of the HF waveguide, leading to a horn 20. Thus, essentially, the MF cone-shaped diaphragm 11 acts as a low Q conical waveguide for the HF acoustic signals. The conventional coaxial transducer 10 constructed in this manner, however, suffers from inherently low Q because it cannot be successfully loaded to a horn 20 for the following reason.
A classic horn design rule, well known in the art, requires that the horn curvature angle should always increase along the path of the horn. As shown in FIG. 1, simply loading the coaxial transducer 10 to the horn 20 would break this rule. Specifically, although the initial horn 18 and the cone-shaped diaphragm 11 expand at an increasing rate (e.g., from A=9°, B=23°, and to C=58° in the illustration), the rate of expansion decreases at the throat (or the rear end) 20a of the horn 20 (from C=58° to D=27° in the illustration). It would be intuitively obvious to one skilled in the art that this design would cause significant reflections off the walls of the horn 20, causing the acoustic signals to arrive at an observer (listener) at multiple times, thereby destroying the temporal coherence of the original signals and further creating various attendant problems (e.g., side lobes and transient smearing).
The present invention is directed to loading a coaxial transducer to a common horn, without disturbing the temporal coherence of the original signals, thus preventing multiple arrival times of signals and any other interference issues. In view of the challenge discussed above, a need exists for a way to load a coaxial transducer to a common horn which provides for (1) constant expansion of a waveguide for acoustic signals, and hence (2) temporal coherence of acoustic signals.